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1. Field of the Invention
The present invention relates to medical devices, and in particular, to cooling mechanisms for cryogenic devices.
2. Background of the Invention
Catheter-based devices for use in surgical procedures and other medical applications are becoming well known. Recently, the use of low temperature fluids, or cryogens, with such catheters to cold-treat target areas has begun to be explored.
The application of cold to selected body tissues provides a number of advantages over prior catheter devices which alternatively use heat, RF energy, laser light, or other means for treating targeted tissue. A device uses the energy transfer derived from thermodynamic changes occurring in the flow of a cryogen through the device. This energy transfer is then utilized to create a net transfer of heat flow from the target tissue to the device, typically achieved by cooling a portion of the device to very low temperature through conductive and convective heat transfer between the cryogen and target tissue.
Structurally, cooling of the device can be achieved through injection of high pressure cryogen through an orifice into an enclosed expansion chamber. Because the cryogen is supplied at high pressure, ranging up to 800 psia, it is generally a liquid-vapor mixture as it travels through the device to the expansion chamber. Upon injection into the expansion chamber, the cryogen undergoes two primary thermodynamic changes: (i) expanding to low pressure and temperature through positive Joule-Thomson throttling, and (ii) undergoing a phase change from liquid to vapor, thereby absorbing heat of vaporization. The resultant flow of low temperature cryogen through the expansion chamber acts to absorb heat from the target tissue and thereby cool the tissue to the desired temperature.
As is well known in the art, of the two processes contributing to the cooling power of the device, evaporative boiling through a change in phase creates a far greater cooling effect through the absorption of latent heat of vaporization, on a specific basis, than merely that of Joule-Thomson cooling alone. Therefore, it is highly desirable to supply the device with a cryogen that is as much in liquid rather than gaseous phase, before the fluid is injected into the expansion chamber to cool tissue. Unfortunately, during transit to the expansion chamber, such as through an elongate catheter, the cryogen supplied typically passes through a region of comparatively high temperature, such as a region of the human body preceding the target area, and is thereby warmed. This warming coupled with head losses in the flow of cryogen down a length of several hundred diameters of tubing, acts to degrade the quality of cryogen from its high pressure liquid form, to a lower pressure, higher temperature, mixed phase form, leading to significantly degraded cooling power of the device. Furthermore, vapor bubbles may form in the injection line, disrupting the smooth flow of cryogen. As is well known to those skilled in the art, the additional adverse effects of sputtering, turbulence, cavitation, and unsteady flow all degrade cooling power.
It is therefore desirable to provide a device which maximizes the cooling power of the flow of cryogenic fluid therethrough, namely through maintaining a steady, uniform supply of high pressure cryogen in liquid phase. It is also desirable to provide a medical device which minimizes cooling losses in the flow of cryogen as it is applied to tissue, as well as maximizing the ratio of the cooling power of the device versus its internal flow lumen diameter.
The present invention provides a medical device to cold treat desired regions. The device includes an injection tube member defining an injection lumen therein. The injection tube member includes a proximal end, an open distal end, and at least one aperture proximate the distal end. A second cooling member is disposed around the injection tube member, defining a cooling lumen therebetween. A third outer tube member is disposed around the second cooling member, defining a return lumen therebetween. A first fluid pathway is thereby provided for fluid to flow from the injection lumen, through to the aperture in the injection tube, and thereafter through the cooling lumen. A second fluid pathway is provided for fluid to flow from the injection lumen, through the distal end of the injection tube, and thereafter through the return lumen. The device may be coupled to a supply of fluid regulated by a controller mechanism to provide for a pressure gradient throughout the first and second fluid pathways. The flow of fluid through the first fluid pathway insulates and cools the fluid supplied into and flowing through the injection lumen. The flow of fluid through the second pathway cools the surrounding areas external to and proximate the distal end of the device.